Film-type evaporators having the aforementioned construction are known in numerous different forms (DE No. 24 09 502, DE No. 1 114 783, U.S. Pat. No. 3,430,690, U.S. Pat. No. 4,167,454). They are more particularly used in the chemical, food and pharmaceutical industries for the distillation and concentration of thermally sensitive products. For boiling point reduction purposes they are often operated with a vacuum. The starting product which is fed onto the evaporating surface close to the rotor axis, spreads outwards in the form of a film due to the centrifugal force. The boiling component evaporates in the vapor chamber, while the concentration in the film increases to the outside. On the outside the concentrate is removed, but is optionally also supplied to a second, similar evaporating stage. The heating chamber of each evaporating surface is connected to its own heating circuit, which e.g. operates with heating steam.
The heat transfer in the liquid film, which moves in a laminar or pseudolaminar flow, essentially takes place through pure heat conduction with surface evaporation. This is helped by the fact that in the centrifugal field the film is under a slight overpressure. The film thickness and residence time of the product on the evaporating surface can be influenced by a rotor speed change and in particular the residence time can be shortened to a few seconds compared with film-type evaporators having fixed evaporating surfaces, which leads to a particularly careful treatment and limited quality reductions in the concentrate. High viscosity liquids can also be processed on the rotating evaporating surfaces.
The known film-type evaporators have conical evaporating surfaces on which the product rises to the outside. Limits are placed on the rotor speed by the fact that the film must not break or tear open, because otherwise caking and encrustation occurs. Further limiting factors are the feed quantity, the thickening ratio of the starting product to the concentrate and the cone angle of the evaporating surface. Conical evaporating surfaces suffer from the fundamental disadvantage that with increasing diameter the circumferential length and therefore the evaporating surface increases, so that towards the outside the film becomes increasingly thin and easily tears. If the product contains particles, the film can easily tear in the vicinity thereof. At the points where the evaporating surface is not covered by a film, the wall temperature rises, which can in turn lead to produce damage.
The problem of the present invention is to so develop a film-type evaporator with the construction described hereinbefore, that a constant quality in the concentrate is obtained and irregularities in the evaporation process are avoided.
According to the invention this problem is solved in that the evaporating surface is a curved blade surface, which is formed by a generatrix substantially parallel to the rotor axis and whose axial extension is limited by upper and lower stop ledges and that the concave side of the evaporating surface points in the circumferential direction.
Unlike in the case of the prior art film-type evaporators, in which the evaporating surfaces are constructed as solids of revolution with the rotor axis as the axis of symmetry, a completely new way is adopted by the invention. The blade-like evaporating surface is formed by an axially parallel generatrix. This means that the evaporating surface can always have the same axial extension from the inside to the outside, i.e. the evaporating surface does not increase from the inside to the outside. The thickness of the film spread on the evaporating surface is only reduced by the evaporation rate and can consequently be very accurately maintained or easily controlled by the feed quantity and/or the speed without other disturbing limiting factors. An uncontrolled spreading in the axial direction is not possible as a result of the centrifugal field and is prevented by the axial stop ledges or strips. Therefore the product is carefully and uniformly concentrated. Caking, encrustation and local overheating are completely avoided. These positive effects are assisted by the fact that the accelerative forces occurring during rotation press the film onto the evaporating surface and consequently there is a pressure rise in the film. This in particular suppresses bubble and froth formation.
In a preferred construction the evaporating surface is corrugated, the corrugations running in axially parallel manner. This improves the heat exchange and stability of the film, there is a certain holding back of the outwardly accelerated film and the evaporation rate on the tops of the corrugations or waves is increased as a result of the small film thickness there.
The evaporating surface forms part of a spiral surface emanating from the rotor axis. If the curvature corresponds to a logarithmic spiral, then the acceleration of the film at all points and also the pressure rise in the film is the same as for conical surfaces. In the case of particularly sensitive products, the axial width of the evaporating surfaces decreases outwardly from the rotor axis. Thus, over the entire evaporating surface an identical film thickness can be maintained, in that the evaporation rate is compensated by the outwardly decreasing evaporating surface.
According to a further advantageous development the heating chamber is bounded by the evaporating surface, a heating chamber wall running substantially parallel to its rear surface and the axial stop ledges. If heating steam is used as the heat carrier, as a result of the following heating chamber the condensate is driven to the outside and can be removed there.
A preferred embodiment results from the fact that the heating chamber wall is corrugated parallel to the stop ledges and that the evaporating surface with its corrugation tops facing the heating chamber is supported on the corrugation tops of the heating chamber wall facing it. As a result of this the evaporating surface can be constructed in a thin-walled manner. As a result of its corrugation, in conjunction with the corrugated heating chamber wall, it still forms a constructional unit stable under high speeds.
Appropriately the evaporating surface and the heating chamber wall are connected at their inner and outer ends with in each case an axially parallel tube, provided on the portion of its casing facing the heating chamber with openings and the tube located closer to the rotor axis is used for supply and the other tube for the discharge of the heating medium. The supply and discharge tubes for the heating medium also constitute a constructional part contributing to the stability of the evaporating surface and which are also used for the fastening thereof.
According to a further feature of the invention several blade-like evaporating surfaces with the same curvature and the heating chambers associated therewith are uniformly distributed in a radial plane of the rotor and several evaporating surfaces and heating chambers can be stacked in axially succeeding manner.
Thus, per space unit, compared with conventional film-type evaporators, a much larger evaporating surface can be housed and consequently the specific evaporating efficiency is much higher.
An advantageous construction is obtained if the rotor has a base with a hub for a drive shaft and axially spaced from said base a spider and between which are positioned the evaporating surfaces. As a result the rotor is a dimensionally stable structure, which is further stabilized in that the evaporating surfaces and the heating chambers associated therewith are fixed to the base by the tubes supplying and removing the heating medium and also fixed in the rotor by the spider projecting over them at the other end.
In order to rapidly remove the concentrate from the rotor without any further thermal loading, at their radially outer ends, where they are externally connected to the tube removing the heating medium, the evaporating surfaces run in channels, whose bottoms, from the base of the rotor to the spider have an increasing spacing from the rotor axis and that all the channels at their ends facing the base of the rotor terminate in a ring channel from which the concentrate collecting therein is removed by means of a skimming tube. Therefore the channels and the evaporating surfaces are readily accessible for maintenance and inspection purposes. The function of draining off the concentrate into the ring channel can also be assumed by a closed, conical rotor casing in place of the individual channels.
The supply of the liquid starting product to the evaporating surfaces, according to a preferred embodiment, takes place in that concentrically to the rotor axis a cylindrical ring channel extending over the axial extension of the evaporating surfaces is provided and its open side is directed towards the rotor axis and to which the starting product is supplied in axially distributed manner by means of a nozzle tube and that the evaporating surfaces are attached externally and in spaced manner to the ring channel and the latter has in the vicinity of the attachment points openings through which the starting product passes from the ring channel to the evaporating surfaces.